In recent years, biological filtration has been increasingly used for cleaning gas and liquid streams originating from various industrial activities like production processes, waste water transport, waste water treatment and others. In biological methods, the neutralization of undesirable components of a gas or a liquid takes place by activity of microorganisms.
In biological gas treatment systems, mesophilic microorganisms are normally used. Mesophilic microorganisms grow in the temperature range of from about 10.degree. C. to about 40.degree. C. Thermophilic microorganisms, which grow in the temperature range of from about 40.degree. C. to about 70.degree. C., may also be used, but until now no effective commercial applications in gas treatment has been available; current reactor designs are not suitable because of the high growth rate of the thermophilic microorganisms or require an expensive multiple stage design.
Known biological gas purification systems comprise biofilters wherein the microorganisms grow on a matrix of wetted packing material, typically organic. As the gas is forced to pass through the packing material, contaminants from the gas are absorbed by the biofilm, which is a layer of water with microorganisms adsorbed on the packing material. The contaminants are essentially oxidized to carbon dioxide and water by microbial activity. Due to nutrient and/or space limitations in the matrix, no significant excess of additional biomass is formed, and the actual purification process by biological degradation of contaminants is limited, resulting in a reduced oxidation capacity. A biofilter is not suitable for thermophilic microorganisms, since these are capable of degrading organic packing material, leading to replacement of the packing material. The packing material is generally kept wet by a saturated gas flow and/or occasional spraying; a biofilter does not make use of a permanent liquid flow. The gas purification method making use of biofilters mostly is at least a two-stage process: first the gases are pretreated, particulate contaminants which may clog the matrix or packing material are removed, the pretreated contaminated gas is saturated with water, and finally treated in the actual biofilter. In this respect, reference is made to U.S. Pat. Nos. 4,662,900 and 4,806,148 and corresponding European Patent No. 0 142 872 and related prior art.
Another known biological gas purification system comprises so-called biotrickling filters (BTF). In the case of a BTF, the microorganisms grow on a packing material in the form of a biofilm. The contaminants from the gas are absorbed into the liquid that drips along the biofilm and diffuse into the biofilm where they are degraded by the microorganisms. The microorganisms and the degradation products such as water, carbon dioxide and occasionally mineral salts or acids are relinquished to the liquid. This liquid will moreover contain nutrients for the microorganisms in addition to acid or alkali to buffer the liquid to a neutral pH. Excessive growth of the biofilm can result in an increased pressure drop over the BTF and ultimately even lead to blocking of the BTF. Several mechanical or chemical procedures are applied to remove at least a part of the biofilm. Also methods are used to limit the growth of the microorganisms beforehand. Reference is made to U.S. Pat. No. 5,637,498 and corresponding PCT Publication No. WO 94/26392 and related prior art. These methods, however, generally reduce the effectiveness of the BTF. A BTF is not suitable for thermophilic microorganisms, as the high growth rate will further increase the clogging problems described previously. Similar to the biofilter, the BTF often is a two-stage process: particulate contaminants which are present in the contaminated gas flow and which may clog the matrix need to be removed before the gas flow enters the BTF.
Yet another known biological gas treatment system comprises a biological scrubber. In this case, the gas contaminants are first absorbed in a scrubbing liquid (absorption stage), generally in a packed bed. The liquid having absorbed therein at least part of the contaminants from the gas flow is collected in a tank from which it is recycled to the absorption stage. In order to keep the concentration of the contaminants in the liquid below their saturation level, part of the liquid is passed over a separate water treatment plant (treatment stage). Preferably, the absorption stage should remain free from microorganisms in order to enhance the transfer of the contaminants and prevent clogging problems. In the treatment stage the contaminants are partially transformed into biomass (microorganisms) and partially into water and carbon dioxide. The excess biomass needs to be removed from the system, more particularly from the scrubbing liquid, to a large extent before such scrubbing liquid is returned to the adsorption stage. When particulate contaminants are present in the gas flow, it is preferable to use a Venturi scrubber in order to best remove such particulate contaminants. In a Venturi scrubber, the contaminated gas is accelerated or injected into the liquid; but like most other known technologies to remove particulate contaminants from a gas flow, a Venturi scrubber has poor absorption capability for contaminants. For this reason the removal of particulate materials mostly is effected in a separate step upstream of the packed bed absorption column. This makes the bioscrubber a complex and expensive three-stage system.
A common problem to all three types of known biological gas treatment systems is that the packed beds or columns used for the biological treatment easily clog due to the increase of biomass and, in the case of the presence of particulate contaminants, due to such particulate contaminants.